This invention relates to an image forming apparatus provided with an image erasing lamp with which a portion of the image can be erased by removing charges from a part of its photosensitive drum such that toner is not attached thereonto.
With an image forming apparatus such as an electrophotographic copier, the surface of a photosensitive drum comprising a photoconductive layer is electrostatically charged first and a latent image is formed thereon by exposing this surface to a beam of light reflected from an original document to cancel the charges from exposed areas. Thereafter, toner is attached to the areas which are still charged, or were not exposed to light, to develop the electrostatic latent image into a visible image and a printed image is finally obtained by transferring this toner image onto a sheet of copy paper. In other words, toner does not become attached to areas on the photosensitive drum which have been exposed to light to have the electrostatic charges cancelled thereon. Thus, if an image erasing lamp (also referred to as blanking lamp) is incorporated in an image forming apparatus between a charger for electrostatically charging its photosensitive drum and a developing device for applying toner onto the surface of the drum, and if a specified area of the drum is exposed to light from the image erasing lamp in addition to the usual exposure of the drum surface to the reflected light from a document to be copied, the charges in such a specified area become completely cancelled from the light from the erasing lamp and toner does not attach thereonto. This principle has already been applied for trimming and masking modes of operation. Trimming is effected by exposing only the area outside a specified closed boundary to light such that image is formed only inside this boundary. Masking is effected by exposing only the area inside a specified closed boundary such that image is formed only outside this boundary.
With electrophotographic copiers of a conventional type, however, position can be specified only for a single closed boundary and the entire area inside or outside this boundary is completely cancelled. Thus, a frame-shaped region, which may be described as being outside one closed boundary and also inside another closed boundary, cannot be specified easily for trimming or masking by a conventional copier. One of the methods of specifying such a frame-shaped region to cancel the charges therein would be to divide the region into four rectangular subregions a, b, c, and d as shown in FIG. 6 and to specify each subregions successively to erase the image therein.